No need for dark energy , gravity will suffice.

AI Thread Summary
The discussion centers on the necessity of dark energy in explaining the universe's expansion, with one viewpoint arguing that gravity alone suffices to account for observed phenomena. It references Hubble's 1929 observations of redshift in galaxies, suggesting that the universe's structure and inevitable collapse can be explained without dark energy. Another perspective highlights that the observed acceleration of galaxy separation contradicts older models, indicating that something, like dark energy, is actively driving this expansion. The conversation also touches on the gravitational interactions among galaxies and the implications of various cosmological models, emphasizing the need for a robust mathematical framework to support claims about the universe's behavior. Ultimately, the debate reflects ongoing inquiries into the fundamental nature of cosmic expansion and the forces at play.
  • #51


Chalnoth said:
Are you somehow thinking about this curvature issue? The open/closed/flat stuff? That doesn't change with time. The density changes, but the expansion rate changes to compensate. So the overall spatial curvature doesn't change.

Isn't it so that any spatial curvature present (not precisely flat) will evolve away from flatness as time goes on? It is true that open will always remain open and closed will always remain closed, but not at the same \Omega, not so?
 
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  • #52


Jorrie said:
Isn't it so that any spatial curvature present (not precisely flat) will evolve away from flatness as time goes on? It is true that open will always remain open and closed will always remain closed, but not at the same \Omega, not so?
Well, it does depend upon how you write down the parameters. Take a look at the Friedmann equation with curvature:

H^2(a) = H^2_0 \left(\frac{\Omega_m}{a^3} + \frac{\Omega_r}{a^4} + \frac{\Omega_k}{a^2} + \Omega_\Lambda \right)

Written this way, the curvature parameter \Omega_k does not change with a. You can think of this as saying that there's still the same total amount of "k" around, just as there's still the same amount of matter around, no matter how much the universe expands. However, if you look at how the effect of the curvature changes with the scale factor a, it dilutes more slowly than normal matter/dark matter or radiation. This means that even if the density fraction in the early universe was small, it tends to grow very rapidly compared to matter and radiation.

However, if there is a cosmological constant, that doesn't dilute at all, which means that the eventually any amount of cosmological constant will cause the curvature to also dilute away to nothing, leaving the dominant effect the cosmological constant at late times. If there was no cosmological constant (or anything else that acts like one), at late times the curvature would become dominant, as long as it is even slightly non-zero.

This is actually why inflation explains why the curvature was so small in our early universe: inflation behaved very much like a cosmological constant, and so it diluted the curvature away to nearly zero before our region began.
 
  • #53


Chalnoth said:
Well, it does depend upon how you write down the parameters. Take a look at the Friedmann equation with curvature:

H^2(a) = H^2_0 \left(\frac{\Omega_m}{a^3} + \frac{\Omega_r}{a^4} + \frac{\Omega_k}{a^2} + \Omega_\Lambda \right)

Written this way, the curvature parameter \Omega_k does not change with a. You can think of this as saying that there's still the same total amount of "k" around, just as there's still the same amount of matter around, no matter how much the universe expands. However, if you look at how the effect of the curvature changes with the scale factor a, it dilutes more slowly than normal matter/dark matter or radiation...

Thx, that's the way I understood it too: \Omega_k is the present curvature parameter and the a^2 denominator sorts out the evolution of the curvature contribution, if nonzero to start with. But as you said, a dominant \Omega_\Lambda can also drive nonzero curvature to zero, so it is not always driven farther away from zero, as I implied...
 
  • #54


String theory appears to be the only thing that seems to completely exclude the possibility of cosmic collapse or the cyclic model.

Not sot: Heterotic M theory (Horava and Witten) with two parallel branes leads to the ekpyrotic cyclical model proposed by Paul Steinhardt and Neil Turok and explained in the book the Endless Universe, 2007. I'm reading it now and the main idea is that in the last ten years major ideas have emerged to revive cyclic models, a major one being that dark energy evolves over time. It speeds up the expansion rate, as is now being observed, acts as a shock absorber of the type used on automatic door closers, and also slowly decays over time shutting itself off.
Dark energy is not a constant!
 
  • #55


Dark energy could only qualify as a cosmological constant if it existed, and this is very doubtful. When described it is always as an expanding force between galaxies, pushing them apart, much as expanding space is described. But what about galaxies between opposing forces? Picture a single galaxy surrounded by dark energy. Pushed from all sides it would go nowhere. The same would apply equally to a cluster of galaxies or a whole universe of galaxies. The overall effect would be neutral, as it would be with vacuum energy, an earlier attempt to explain galactic movement.
 
  • #56


Peter Watkins said:
Dark energy could only qualify as a cosmological constant if it existed, and this is very doubtful. When described it is always as an expanding force between galaxies, pushing them apart, much as expanding space is described. But what about galaxies between opposing forces? Picture a single galaxy surrounded by dark energy. Pushed from all sides it would go nowhere. The same would apply equally to a cluster of galaxies or a whole universe of galaxies. The overall effect would be neutral, as it would be with vacuum energy, an earlier attempt to explain galactic movement.
By this same logic, nothing could speed up or slow down the expansion. After all, being pushed equally in all directions is identical, in your little thought experiment, to being pulled in all directions. But the expansion rate has changed over time, so obviously something is wrong with your little thought experiment.
 
  • #57


Hello Chalnoth. Re above; it is not a thought experiment, it is a simple statement of fact. Cosmologists seem to have forgotten that there is a fundamental law that applies throughout the universe ie., for every action there is an equal and opposite reaction. With regard to nothing to speed up expansion, you are quite right. It is an impossibility. But the rate at which galaxies move apart will increase as the expansion rate is slowed from within. This would resemble acceleration from our viewpoint. You yourself went to great lengths to impress upon me that gravity will always exert a restraining effect, and indeed it is, slowing the rate at which the universe is expanding. It can also cause acceleration, after first slowing the innermost galaxies to a halt, and then pulling them back at an increasing rate. If the universe was made up of layers, like an onion, it is these that are being separated. The outer layers, including us, are still moving outward at a rate that decreases toward the centre, whilst the inner layers are collapsing inward at a rate that increases toward the centre. This picture fits all observations. So, to state that there is no mechanisms to alter expansion rates is incorrect.
 
  • #58


Peter Watkins said:
So, to state that there is no mechanisms to alter expansion rates is incorrect.
You seem to be very confused. The FRW universe is one wherein the universe is modeled as a perfectly uniform fluid. This is known to be not entirely accurate, but works pretty well on large scales. And yes, a perfectly uniform fluid can expand and contract based upon the action of gravity combined with whatever internal pressures this fluid feels.

You can argue in words all you like, but the mathematics is unambiguous and certain: gravity does propel a changing rate of expansion.
 
  • #59


Why on Earth is the universe modeled as a perfectly formed fluid? I would have thought that a perfectly formed vacuum would be more appropriate. I agree that gravity can alter he rate of expansion, but only by slowing. Cosmology is a science of observation and reason, not mathematics. I'll wager you that mathematics could not begin to describe our solar system to the un-informed, whilst words and illustrations would quickly convey the message.
Believe me, there is no confusion on my part. I thank you for your time.
 
  • #60


Peter Watkins said:
Why on Earth is the universe modeled as a perfectly formed fluid?
Not perfectly formed. Perfectly uniform. That's the FRW universe, anyway. What cosmologists typically work with is perturbed FRW, where that assumption of uniformity is relaxed.

But regardless, the reason why it's modeled that way is because it's a good approximation.

Peter Watkins said:
I agree that gravity can alter he rate of expansion, but only by slowing.
Well, you'd be wrong. It depends upon the matter/energy content of the universe.
 
  • #61


Peter Watkins said:
Cosmology is a science of observation and reason, not mathematics.

Since you're not a cosmologist, it would be wise of you to refrain from telling them how they should conduct their research.

The simple fact is that physics without mathematics is no longer physics. It is philosophy at best, and pointless word games at worst. Physical theories cast in language alone are far too vague and ambiguous to ever make concrete predictions.

Your comments on pictures and words making for better pedagogy are misguided, but even if they were correct, they would be irrelevant. We're not talking about teaching cosmology; we're talking about understanding the universe with a meaningful theory that makes meaningful predictions. Scientists would be enormously handicapped if they chose to only explore theories which are accessible to a lay audience.

- Warren
 
  • #62


The universe is essentially a study of the large and the small. Virtually all of the mystery is contained within the atom, how and why they form, interactions, component parts, why there are any forces and so on. And for this mathematics are essential. Then there is the large scale universe. The different types of stars, their life cycles, pulsars, neutron stars, variables etc.. It has taken the finest brains to tease out the secrets of these objects and I don't doubt for a moment that mathematics were involved. But when it comes to large scale movements, what we have is large clumps of matter, inertial or kinetic energy, and gravity. A "lay" person does not need to know how or why gravity works, simply what it does.
This forum constantly denies that the universe expanded from a single point, but we "lay" people have been told for 80 years that the universe is expanding, that is, increasing in size, in all directions, for close to 14 billion years. So, unless I'm living in a crazy alternative reality, this can only mean that the universe was considerably smaller than it now is, probably by a factor of several billions. Regardless of whether this was as small as the volume of our solar system, or our galaxy, or our local group, the fact is that the expansion took place, from that point, against the restraint of gravity. So what is needed is the imagination to picture the manner in which it would expand. For this, a knowledge of mathematics would seem unnecessary. What I don't understand is why this forum is so set against the notion that gravitational restraint from within has, or could have, caused "spreading", ie. the "faster with distance" view, and that this process is continuing, producing the illusion of acceleration. Surely the last thing a cosmologist should possesses is a closed mind.
Re post #60. Matter and energy are two sides of the same coin, the one always resulting only from the other, either from heating or cooling. Which, as an aside, is worth remembering when trying to fathom our origins.
 
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  • #63


Peter Watkins said:
So what is needed is the imagination to picture the manner in which it would expand. For this, a knowledge of mathematics would seem unnecessary.
And that's where you're wrong. Only mathematics is precise enough a language to determine what is or is not occurring.
 
  • #64


Chalnoth said:
And that's where you're wrong. Only mathematics is precise enough a language to determine what is or is not occurring.

I disagree. Pig Latin has been demonstrated to be a much more efficient and descriptive language for describing the inner workings of the universe.

Ask anyone. It's science.
 
  • #65


Science without math is philosophy. Philosophy is not bad, merely incomplete. Math is unforgiving and rigorous. It's what debunks the process of knowledge. Nature does not prohibit an illogical universe. It does prohibit a mathematically inconsistent universe.
 
  • #66


Chronos said:
Nature does not prohibit an illogical universe. It does prohibit a mathematically inconsistent universe.
Only if you use an improper definition of the word "illogical". Formally, illogical = mathematically inconsistent, as formal logic is a type of mathematics. Perhaps "unintuitive" would be a better word: there is no reason why the universe shouldn't be unintuitive.
 
  • #67


This whole topic is fascinating. I'm not well versed in field but I appreciate the insight and the references. I'm curious if anyone has explored the idea of something at the edge of the universe pulling things apart. In effect, a "big rip" instead of a "big bang." That same force could still be there today exerting gravitational pull and causing the increasing speed of separation that leads us to this whole dark energy idea.
 
  • #68


LostUpNorth said:
This whole topic is fascinating. I'm not well versed in field but I appreciate the insight and the references. I'm curious if anyone has explored the idea of something at the edge of the universe pulling things apart. In effect, a "big rip" instead of a "big bang." That same force could still be there today exerting gravitational pull and causing the increasing speed of separation that leads us to this whole dark energy idea.

I've thought about this, too. Is there any reason the universe can be pulled out, instead of pushed out?
 
  • #69


Assuming there is nothing 'outside' the universe, pushing is the only apparent option.
 
  • #70


Chronos said:
Assuming there is nothing 'outside' the universe, pushing is the only apparent option.

Interesting point. I'd be very interested in any work that analyzes whether or not there could be something at the edge or outside of the universe. That analysis may prove or disprove dark energy quite well.

As I said, I'm not well versed in the field, but to me it's simple. If dark energy is acting as some sort of cosmic wind that pushes things in a direction away from a point, then our orbiting galaxies and planets should show some measure of change as they head "into" the wind. Imagine trying to fly an airplane in a circle on a windy day. On the down wind side the plane will go faster and on the upwind side it goes slower. To get a true circle you must constantly adjust the rate of turn. If you keep the same rate of turn your plane will spiral along in ellipses as the wind pushes you farther and farther along.

If indeed there is dark energy pushing our galaxy away from some location at an ever increasing speed, we should be able to measure acceleration AND deceleration within the galaxy as well. Quite simply, the up wind stuff should be going slower than the down wind stuff.
 
  • #71


Nearly all the "idea" of 'dark energy' is based on the apparent acceleration seen when comparing data and models to an 'empty model' -- the Milne model.

Problem is: What if this is simply due to a 'quirk' in the Milne model? It is known to be wrong so why is this 'acceleration hump' being used as a reason for 'dark energy'?

Seems far more likely that its simply a shortcoming of the Milne -- and likely Standard Model.
 
  • #72


Science without math is philosophy. Philosophy is not bad, merely incomplete. Math is unforgiving and rigorous. It's what debunks the process of knowledge. Nature does not prohibit an illogical universe. It does prohibit a mathematically inconsistent universe.

I'm wondering what your distinction is between 'illogical' and 'mathematically inconsistent'? Is this intended to be some 'workaround' for cause and effect?

My view is different. Math is the language of science. When its found that a new 'word' is needed to explain the science -- a new 'word' (or math) is invented or found.

Science is in the concept not the details. Math and data are the way science is 'proved' -- it is NOT the science itself.

Math is NOT a requirement of science. Math is ONLY a more precise language or way of communicating the science.
 
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  • #73


Hal King said:
I'm wondering what your distinction is between 'illogical' and 'mathematically inconsistent'? Is this intended to be some 'workaround' for cause and effect?

My view is different. Math is the language of science. When its found that a new 'word' is needed to explain the science -- a new 'word' (or math) is invented or found.

Science is in the concept not the details. Math and data are the way science is 'proved' -- it is NOT the science itself.

Math is NOT a requirement of science. Math is ONLY a more precise language or way of communicating the science.
This would be all well and good, except that it turns out that pursuit of "mathematical beauty" has been an extraordinarily useful tool in discovering new theories. By mathematical beauty here I mean finding ways to write down the behavior of whatever is being considered in as few mathematical statements as possible.

Then there's the simple argument that a mathematical structure is any structure which is strictly-defined and fully self-consistent. Clearly the universe exists in one way and one way only, and is also self-consistent, so it stands to reason that the universe is, at its heart, mathematical.

Finally, when you stray away from using mathematics to describe the behavior of the universe, you invariably end up with ambiguities and inaccuracies. Plus you can't effectively work out what the consequences of statements about the universe are without mathematics. As a result, one cannot do science without mathematics.
 
  • #74


Reminds me of the old 'every logical or mathematical system is either incomplete or contradictory'.

Math is a minor player in science -- a useful tool -- similar to engineering or taking measurements.
You have to have it, have to use it, but it is NOT the primary focus or goal.

When the math fails the concept there are TWO possibles:

1) Bad concept
2) Bad math

Before you give up on the concept its best to recheck the math -- or even create a new one if needed. Math 'proves' nothing on its own. Might be interesting to some -- even 'beautiful' -- but that is a personal view that does not effect the science.

Also, I don't see why the universe only has to exist in one way? Maybe only be perceived in one way at a time -- again a viewpoint issue.
 
  • #75


Hal King said:
Reminds me of the old 'every logical or mathematical system is either incomplete or contradictory'.

Math is a minor player in science -- a useful tool -- similar to engineering or taking measurements.
You have to have it, have to use it, but it is NOT the primary focus or goal.
Then why has taking it as fundamental proven so useful?
 
  • #76


Didn't say it wasn't useful ... only that the math is not the real point.
 
  • #77


Hal King said:
Didn't say it wasn't useful ... only that the math is not the real point.
That's not what I asked. I didn't ask why mathematics is useful. I asked why taking mathematics as fundamental has proven useful.
 
  • #78


Again that has not been fully determined as yet in cosmology. Might just as well be leading us in entirely the wrong direction.
 
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  • #79


Hal King said:
Again that has not been fully determined as yet in cosmology. Might just as well be leading us in entirely the wrong direction.
It has already been shown to be extraordinarily successful in cosmology, where prediction after prediction has been borne out with stunning precision.
 
  • #80
  • #81


what is the meaning of the nost incomprihensive thing about the universe is that it is comprihensive give detail meanig of it
 
  • #82


The fundamental evidence for dark energy came from the Perlmutter supernova study. See, for example: http://arxiv.org/abs/astro-ph/0309368

Simple geometric corrections using other than Standard Model give better data fits with no acceleration. Of course, that won't stop 'big science'. Too many careers are now tied to the concept. So something that will be given the label 'dark enegy' will eventually be found, no matter what the cost.
 
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  • #83


Hal King said:
Sorry, I've been working on such for the last year ... don't buy it.

Simple geometric corrections using other than Standard Model give better data fits with no acceleration. Of course, that won't stop 'big science'. Too many careers are now tied to the concept. So something that will be given the label 'dark enegy' will eventually be found, no matter what the cost.
Er, this is just false. You may potentially be able to fit some small subset of the data, but you can't match different observations without invoking some sort of modified gravity or dark energy.
 
  • #84


Hal King said:
Simple geometric corrections using other than Standard Model give better data fits with no acceleration. Of course, that won't stop 'big science'. Too many careers are now tied to the concept. So something that will be given the label 'dark enegy' will eventually be found, no matter what the cost.
:zzz:

This kind of qausi-conspiracy theory stuff is just non-sense. Have a look at cosmology pre-prints, http://arxiv.org/list/astro-ph.CO/recent". You will find a very active field with all kinds of different new ideas looking at dark energy and ways to fit the data with all kinds of other theories. The idea that 'big science' is stuck on one idea that it refuses to let go of is simply not supported by the evidence of just looking at the variety of ideas being out there (and not simply ignored).

Anyone working in cosmology would love to find a better theory than what we have at present, I gaurantee you that there is not a single person whose career would not benefit by overturning the current standard model of cosmology, which is why so many people are working so hard to do just that. The fact that the standard model has held up very well despite continued efforts to find problems with it just tells us that something about the model is very powerful, even if ultimately the physics behind it turn out to be rather different from what we currently suspect.

On the other hand, Bad new ideas that are based on simple misconceptions and don't consider the totality of the evidence available are, of course, ignored. That doesn't mean that there isn't a lively and open debate going on with regards to genuinely interesting alternatives.
 
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  • #85


Actually, all you can say for any model is whether it matches data or not. The importance of an 'accepted' model is that it determines what will be used for further work ... grants, money, careers, promotions, etc. So if you are 'in the business' you have to use the accepted approaches.
 
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  • #86


Hal King said:
Actually, all you can say for any model is whether it matches data or not. The importance of an 'accepted' model is that it determines what will be used for further work ... grants, money, careers, promotions, etc. So if you are 'in the business' you have to use the accepted approaches.

:rolleyes: Again, this is unfounded non-sense. A plurality of approaches are being pursued and jobs, grants and funding are flowing to all kinds of ideas. I have no idea how you think the system works, but it's nothing like you are suggesting. If anything, it remains easier to get funding for claiming you will pursue some great new idea. Re-iterating the status quo is boring, it doesn't excite the researchers in the field or funding agencies. That being said, even the 'standard' model is such a bare boned sketch at present that there are plenty of interesting things still to be worked out within the context of the 'standard' approach. This is also a very active research area, since it is only by working out the details that we will be able to see if the model eventually stands up to scrutiny or whether significant problems are revealed.

You can rant all you like, but the fact is that well posed alternative explanations get a fair hearing. The whole dark energy idea was once 'alternative', as were other elements you could now call 'standard'. If an alternative theory works it will become the standard. Nobody has any interest or anything to gain by preventing that.
 

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